This invention relates to certain liquid crystal compounds of the smectic type. More particularly, it relates to smectic liquid crystals which exhibit a ferroelectric, chiral smectic C phase and which find application in electrooptic display devices.
The utilization of the properties of a ferroelectric smectic phase to effect a switching phenomenon in an electrooptic display device has been known and is described by N. A. Clark and S. T. Lagerwall in App. Phys. Lett. 36, 899 (1980). Such devices operate with low electric power consumption while providing a more rapid switching than is realized in electrooptic display devices dependent upon the properties of liquid crystals of the nematic type. Display devices of the ferroelectric type rely upon two tilted configurations of smectic liquid crystal molecules to provide two states of equal energy. Switching from one state to the other is accomplished by moving a boundary between the two domains by applying an electric field across a pair of electrodes sandwiching a layer of the ferroelectric liquid crystal material.
Examples of ferroelectric liquid crystal compounds of the smectic type are described, for example, by G. Decobert and J. C. Dubois in Mol. Cryst. Liq. Cryst., 1984, 114, 237-247; by J. W. Goodby and T. M. Leslie in Liquid Crystals and Ordered Fluids, Edited by A. C. Griffin and J. F. Johnson, Plenum Press, Vol. 4, pp. 1-32; and in European Patent Application 0110299 A2, published Jun. 13, 1984. In the aforementioned European Patent Application 0110299 A2, it is indicated that the appearance of ferroelectricity in a molecular structure is dependent upon two conditions--the presence of an optically active group and the presence of an electric dipole in a direction approximately perpendicular to the major axis of the liquid crystal molecule, to induce spontaneous polarization. In general, it is well recognized that the suitability and operating efficiency of an electrooptic device will be dependent upon the chemical structure of a liquid crystal compound employed therein and that such properties will be influenced by such molecular factors as rigidity or stiffness, morphology, crystallinity and intermolecular forces.
In the production of ferroelectric liquid crystal compounds of the smectic type for application in ferroelectric devices, it will be advantageous if the ferroelectric liquid crystal material shows a smectic phase over a wide range of temperatures including room temperature. Accordingly, there is an interest in ferroelectric, smectic liquid crystal compounds and compositions which can over a range of temperatures be employed in an electrooptic device for high-speed optical switching, particularly at room temperature.